Airfoils for gas turbine engines are disposed in a flowpath for working medium gases. Examples of such airfoils are turbine blades and turbine vanes. The airfoils are bathed in hot gases as the gases are flowed through the engine. Cooling air is flowed though passages on the interior of the airfoil under operative conditions to keep the temperature of the airfoil, such as a turbine vane or turbine blade, within acceptable limits.
In addition, the airfoil may have cooling air holes extending from the interior to the exterior of the airfoil. The cooling air holes duct cooling air from passages on the interior of the airfoil through the hot walls to the exterior. The exhausted cooling air provides transpiration cooling as the air passes through the wall and film cooling with a film of cooling air on the exterior as the air is discharged from the airfoil. The film of cooling air provides a barrier between the airfoil and the hot, working medium gasses.
The cooling air holes are small and may have diameters that are in a range of eleven to seventeen mils (0.011-0.017 inches). The holes are drilled in predetermined patterns and are contoured to insure adequate cooling of the airfoil.
One way to drill the holes uses a laser to direct a beam of coherent energy at the exterior of the airfoil. The intense radiation from the laser beam bums through the wall of the airfoil, leaving behind a hole which provides a satisfactory conduit for cooling air. As the laser beam penetrates through the airfoil wall into an interior cavity, the laser beam may strike adjacent structure on the other side of the cavity causing unacceptable damage to the airfoil. Accordingly, blocking material may be disposed in the cavity to block the laser beam from striking walls bounding the cavity after the beam penetrates through the airfoil wall.
One approach is to leave disposed within the airfoil the ceramic casting core around which the blade is poured during the manufacturing process. The ceramic core provides a suitable blocking material. The ceramic core is subsequently removed by well known leaching techniques. This approach is described in U.S. Pat. No. 5,222,617 entitled "Drilling Turbine Blades" issued to Gregore, Griffith and Stroud. However, the presence of the core after casting prevents initial inspection of the interior of the airfoil. The ceramic material may also be difficult to remove once the cooling air holes are drilled. In addition, the core is not available during repair processes for the airfoil which may require redrilling of the cooling air holes.
Another example of a blocking material is wax or a wax-like material. The material is melted so that it may easily flow into interior passages, such as the leading edge passage of the airfoil. The temperature of the molten material above its melting point, may exceed two hundred and fifty degrees Fahrenheit (250.degree.). The molten material may be poured or injected into the cavity or may even be sprayed or painted on the surface to be protected. However, the molten material may severely scald personnel working with the material. In addition, the wax may extend between two closely adjacent cooling air holes. The wax adjacent the first hole, which blocks the laser beam as the second hole is drilled, may melt as the first hole is drilled by the laser beam. This causes a void to form in the wax. As a result, the energy from the laser beam at the second hole may not be sufficiently dissipated by the wax as it passes through the portion of the passage having the void. Damage may occur to the airfoil as the second hole is drilled because the beam, after it penetrates through the wall at the second hole, may strike the interior wall of the airfoil.
One wax-like blocking material which uses an additive to avoid forming voids is discussed in U.S. Pat. No. 5,049,722, issued to Corfe and Stroud, entitled "Laser Barrier Material And Method Of Laser Drilling." In Corfe, a PTFE (polytetra fluoroethylene) wax-like material is disposed in a wax base. The PTFE helps avoid the formation of voids. Disposing such material on the interior of a leading edge passage is particularly difficult for some airfoils. Often the leading edge passage has no connection during fabrication with the exterior of the airfoil. It is a blind or dead end passage prior to the drilling operation except for small impingement holes which place the passage in gas communication with an adjacent passage. The adjacent passage also has an opening for receiving cooling air which is flowed to the leading edge passage. Accordingly, personnel must carefully pour the molten material in the inlet opening and manipulate the airfoil to avoid bubbles in the material in the leading edge passage.
Still another approach is to use a masking agent, such as an epoxy resin, which is disposed in the airfoil in a fluid state. The epoxy resin is disposed in the airfoil by simply pouring the resin into the airfoil. The epoxy resin is at room temperature and poses no scalding hazard to personnel. The epoxy resin is further processed to harden the fluid and cause it to become a more solid material similar to the PTFE wax mentioned in U.S. Pat. No. 5,049,722. However, the resin is relatively viscous compared to molten wax and has difficulty in flowing through small connecting passages on the interior of the airfoil.
As a result, air bubbles tend to form in blocking material in blind cavities, such as the leading edge passage, and in passages which are blocked by the in flowing viscous material. These air bubbles result from not flowing enough epoxy resin into the leading edge region to eliminate the voids. In addition, the structural design of the airfoil may cause the airfoil to trap an air bubble with structure within the leading edge passage.
Accordingly, scientists and engineers working under the direction of Applicants' Assignee have sought to develop a method for disposing fluid blocking material in a structure having a shaped cavity such as the leading edge of an airfoil such that the fluid material in the leading edge cavity is hardened without forming voids in the cavity that would cause unacceptable damage to structure bounding the cavity during laser drilling.